(a) Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a vertical alignment liquid crystal display which has a structure capable of dividing a pixel area into multiple domains with different orientation directions of liquid crystal molecules.
(b) Description of the Related Art
Generally, liquid crystal displays have a structure where a liquid crystal is sandwiched between two substrates, and an electric field is applied to the liquid crystal display to control the amount of light transmission.
In the twisted nematic (TN) liquid crystal displays, the liquid crystal molecules injected into the gap between the two substrates are oriented parallel to the substrates, and spirally twisted with a predetermined pitch. The long axis (usually called the “director”) of the liquid crystal molecules continuously varies in orientation direction, and the viewing angle characteristics depend upon such orientation directions of the liquid crystal molecules
However, the TN liquid crystal display, light is not completely blocked at an off state in the normally black mode so that a poor contrast ratio results. The contrast ratio is altered depending upon the viewing angle, and there is a half tone of brightness difference depending on the viewing angle so that stable picture images cannot be obtained. Furthermore, the picture qualities at side edge portions of the screen are not symmetrical to each other with respect to the middle portion. These all result in poor viewing angle characteristics.
However, the vertical alignment liquid crystal displays where the liquid crystal molecules are vertically aligned in the absence of a voltage but twisted in various directions with the voltage applied exhibit excellence in various aspects, such as contrast ratio and response speed compared to the TN liquid crystal displays. Furthermore, when a compensation film is used to divide the twisting of the liquid crystal molecules in various predetermined directions, a wide viewing angle can be effectively obtained.
Recently, a technique for forming an alignment control member such as a pyramid-shaped protrusion on the substrates, a technique for forming an opening pattern at the transparent electrodes, and a technique for forming a protrusion pattern on one of the substrates while forming an opening pattern at the other substrate have been proposed as methods to control the orientation directions of the liquid crystal molecules. The protrusion or opening pattern is designed to achieve four domain divisions in the orientation direction of the liquid crystal molecules at which the efficiency of light usage becomes highest.
FIGS. 1A and 1B are cross sectional views of a liquid crystal display according to a prior art where the orientation states of liquid crystal molecules are illustrated in the absence and presence of the voltage application.
As shown in the drawings, a transparent pixel electrode 11 is formed at a bottom substrate 10, and a first opening portion 1 is formed at the pixel electrode 11. A top substrate 20 facing the bottom substrate 10 is provided with a transparent common electrode 21. A second opening portion 2 is formed at the common electrode 21. The bottom and top substrates 10 and 20 are arranged such that the first opening portion 1 is displaced with respect to the second opening portion 2. Negative dielectric anisotropy liquid crystal molecules 30 are injected into the gap between the bottom and top substrates 10 and 20.
As shown in FIG. 1A, the liquid crystal molecules 30 are oriented perpendicular to the substrates 10 and 20 in the absence of the voltage application.
As shown in FIG. 1B, when voltage is applied to the pixel electrode 11 and the common electrode 21, most of the regions at the pixel area are under the influence of an electric field normal to the substrates 10 and 20, but the regions adjacent to the opening portions 1 and 2 are under a fringe field beginning from the edges of the opening portions 1 and 2 and focused onto the common electrode 21 and the pixel electrode 11, respectively. As the negative dielectric anisotropy liquid crystal molecules 30 are inclined to orient in a direction normal to that of the electric field, the long axes of the liquid crystal molecules adjacent to the opening portions 1 and 2 are twisted while being tilted with respect to the substrates 10 and 20. In this case, two side regions of either of the opening portions 1 and 2 where the orienting directions of the liquid crystal molecules 30 are opposite to each other are present so that the optical characteristics of the two side regions are compensated, resulting in a wide viewing angle.
FIGS. 2A and 2B are cross sectional views of a liquid crystal display according to another prior art where the orientation states of liquid crystal molecules are illustrated in the absence and presence of voltage application.
As shown in FIGS. 2A and 2B, a pixel electrode 12 based on a transparent conductive material such as indium tin oxide is formed at a bottom substrate 10, and a pyramid-shaped first protrusion 13 and a vertical alignment film (not shown) are sequentially formed on the pixel electrode 12. A transparent common electrode 22 is formed at a top substrate 20, and a second pyramid-shaped protrusion 23 and a vertical alignment film (not shown) are sequentially formed at the common electrode 22. Negative dielectric anisotropy liquid crystal molecules 30 are injected into the gap between the vertical alignment films of the bottom and top substrates 10 and 20.
As shown in FIG. 2A, in the absence of voltage application, most of the liquid crystal molecules 30 are oriented perpendicular to the vertical alignment films, but the liquid crystal molecules 30 positioned close to the protrusions 13 and 23 are tilted with respect to the vertical alignment films at predetermined angles.
As shown in FIG. 2B, when voltage is applied to the pixel and common electrodes 12 and 22, the liquid crystal molecules are twisted in a direction parallel to the substrates 10 and 20. As the liquid crystal molecules 30 positioned close to the protrusion 13 are tilted in the opposite directions with respect to the region of the protrusion 13 in the absence of the voltage application, the twisting directions thereof are also opposite to each other with the voltage applied. Therefore, two side regions of the protrusion 13 where the twisting directions of the liquid crystal molecules are opposite to each other are present so that the optical characteristics of the two regions are compensated, resulting in a wide viewing angle. In addition, disclination regions where the orientation directions of the liquid crystal molecules 30 are disorderly altered are focused at the regions of the protrusions 13 and 23 so that a black matrix for shielding the disclination regions can be formed in a predetermined manner.
However, in order to fabricate the above-described liquid crystal displays, additional processes for forming the protrusions 13 and 14 or the opening portions 1 and 2 must be performed.
On the one hand, in the case of the liquid crystal display shown in FIGS. 1A and 1B, a wet etching process for forming the opening portion 2 at the ITO-based common electrode 21 of the top substrate 20 should be provided. Furthermore, in order to prevent the color filter from being contaminated or damaged due to the etching solution, a protective layer of organic or inorganic materials should be coated onto the color filter before the ITO processing.
On the other hand, in the case of the liquid crystal display shown in FIGS. 2A and 2B, before the formation of the protrusions 13 and 23, separate organic layers should be coated onto the pixel electrode 12 and the common electrode 22, and etched.